FLT3 and FLT3-ITD phosphorylate and inactivate the cyclin-dependent kinase inhibitor p27Kip1 in acute myeloid leukemia

Prognostic significance of p27 in colorectal cancer: a meta-analysis and bioinformatics analysis

Background

In the past, numerous investigations have delved into the influence of p27 (p27kip) on the prognosis and clinicopathological characteristics of colorectal cancer (CRC), yielding conclusions that are not universally statistically significant, thus rendering the discourse rather contentious.

Methods

We collected available articles published before August 2024 and extracted data to analyze the association between the expression of p27 and the prognosis and clinicopathological features of CRC. In addition, we used Gene Expression Profiling Interactive Analysis (GEPIA), University of Alabama at Birmingham's Cancer Data Analysis Portal (UALCAN), and the Human Protein Atlas (HPA) to validate our results.

Results

Through an extensive examination of four prominent databases, a total of 21 original articles encompassing a cohort of 3,378 patients were identified. The findings indicated that a low expression of p27 could lead to shorter overall survival (OS) [hazard ratio (HR) = 0.44, 95% confidence interval (95%CI) = 0.31-0.61, Z = 4.89, p = 0.000] and disease-free survival (DFS) (HR = 0.40, 95%CI = 0.28-0.59, Z = 4.75, p = 0.000). In addition, a low expression of p27 predisposed tumors to the right colon [odds ratio (OR) = 0.61, 95%CI = 0.46-0.82, Z = 3.32, p = 0.001] and limited tumor differentiation (OR = 0.56, 95%CI = 0.41-0.77, Z = 3.62, p = 0.000), but had no effect on TNM staging (OR = 0.80, 95%CI = 0.52-1.22, Z = 1.05, p = 0.295), lymph node metastasis (OR = 0.90, 95%CI = 0.25-3.28, Z = 0.16, p = 0.876), and tumor size (OR = 0.94, 95%CI = 0.54-1.65, Z = 0.21, p = 0.835). The results from GEPIA and UALCAN showed that p27 had no effect on TNM staging, lymph node metastasis, DFS, and OS; moreover, there was no expression difference between tumor tissues and normal tissues. The findings from the HPA indicated that there was lower expression of p27 in tumor tissues compared with normal tissues.

Conclusion

Although inconsistent results were reached with the bioinformatics analysis from this meta-analysis, it was confirmed that a low expression of p27 can adversely affect the prognosis of patients with CRC and make a meaningful impact on a part of the clinicopathological features in the meta-analysis with abundant data. In the future, predicting the prognosis of patients with CRC and guiding treatment might emerge as a significant objective.

Key role of glutamine metabolism in persistence of leukemic cells upon exposition to FLT3 tyrosine kinase inhibitors

Acute myeloid leukemias are a group of hematological malignancies characterized by a poor prognosis for survival. The discovery of oncogenic mutations in the FMS-like tyrosine kinase 3 (FLT3) gene has led to the development of tyrosine kinase inhibitors such as quizartinib. However, achieving complete remission in patients remains challenging because these new tyrosine kinase inhibitors (TKIs) are unable to completely eradicate all leukemic cells. Residual leukemic cells persist during quizartinib treatment, leading to the rapid emergence of drug-resistant leukemia. Given that mitochondrial oxidative metabolism promotes the survival of leukemic cells after exposure to multiple anticancer drugs, we characterized the metabolism of leukemic cells that persisted during quizartinib treatment and developed metabolic strategies to eradicate them. In our study, employing biochemical and metabolomics approaches, we confirmed that the survival of leukemic cells treated with FLT3 inhibitors critically depends on maintaining mitochondrial metabolism, specifically through glutamine oxidation. We uncovered a synergistic interaction between the FLT3 inhibitor quizartinib and L-asparaginase, operating through antimetabolic mechanisms. Utilizing various models of persistent leukemia, we demonstrated that leukemic cells resistant to quizartinib are susceptible to L-asparaginase. This combined therapeutic strategy shows promise in reducing the development of resistance to FLT3 inhibitors, offering a potential strategy to enhance treatment outcomes.

CircRNAs as New Therapeutic Entities and Tools for Target Identification in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a genetically extremely heterogeneous disease. Drug resistance after induction therapy is a very frequent event resulting in poor medium survival times. Therefore, the identification of new targets and treatment modalities is a medical high priority issue. We addressed our attention to circular RNAs (circRNAs), which can act as oncogenes or tumor suppressors in AML. We searched the literature (PubMed) and identified eight up-regulated and two down-regulated circ-RNAs with activity in preclinical in vivo models. In addition, we identified twenty-two up-regulated and four down-regulated circRNAs with activity in preclinical in vitro systems, but pending in vivo activity. Up-regulated RNAs are potential targets for si- or shRNA-based approaches, and down-regulated circRNAs can be reconstituted by replacement therapy to achieve a therapeutic benefit in preclinical systems. The up-regulated targets can be tackled with small molecules, antibody-based entities, or other modes of intervention. For down-regulated targets, up-regulators must be identified. The ranking of the identified circRNAs with respect to therapy of AML will depend on further target validation experiments.

EpoR Activation Stimulates Erythroid Precursor Proliferation by Inducing Phosphorylation of Tyrosine-88 of the CDK-Inhibitor p27Kip1

Erythrocyte biogenesis needs to be tightly regulated to secure oxygen transport and control plasma viscosity. The cytokine erythropoietin (Epo) governs erythropoiesis by promoting cell proliferation, differentiation, and survival of erythroid precursor cells. Erythroid differentiation is associated with an accumulation of the cyclin-dependent kinase inhibitor p27Kip1, but the regulation and role of p27 during erythroid proliferation remain largely unknown. We observed that p27 can bind to the erythropoietin receptor (EpoR). Activation of EpoR leads to immediate Jak2-dependent p27 phosphorylation of tyrosine residue 88 (Y88). This modification is known to impair its CDK-inhibitory activity and convert the inhibitor into an activator and assembly factor of CDK4,6. To investigate the physiological role of p27-Y88 phosphorylation in erythropoiesis, we analyzed p27Y88F/Y88F knock-in mice, where tyrosine-88 was mutated to phenylalanine. We observed lower red blood cell counts, lower hematocrit levels, and a reduced capacity for colony outgrowth of CFU-Es (colony-forming unit-erythroid), indicating impaired cell proliferation of early erythroid progenitors. Compensatory mechanisms of reduced p27 and increased Epo expression protect from stronger dysregulation of erythropoiesis. These observations suggest that p27-Y88 phosphorylation by EpoR pathway activation plays an important role in the stimulation of erythroid progenitor proliferation during the early stages of erythropoiesis.

Overcoming Resistance: FLT3 Inhibitors Past, Present, Future and the Challenge of Cure

FLT3 ITD and TKD mutations occur in 20% and 10% of Acute Myeloid Leukemia (AML), respectively, and they represent the target of the first approved anti-leukemic therapies in the 2000s. Type I and type II FLT3 inhibitors (FLT3i) are active against FLT3 TKD/ITD and FLT3 ITD mutations alone respectively, but they still fail remissions in 30-40% of patients due to primary and secondary mechanisms of resistance, with variable relapse rate of 30-50%, influenced by NPM status and FLT3 allelic ratio. Mechanisms of resistance to FLT3i have recently been analyzed through NGS and single cell assays that have identified and elucidated the polyclonal nature of relapse in clinical and preclinical studies, summarized here. Knowledge of tumor escape pathways has helped in the identification of new targeted drugs to overcome resistance. Immunotherapy and combination or sequential use of BCL2 inhibitors and experimental drugs including aurora kinases, menin and JAK2 inhibitors will be the goal of present and future clinical trials, especially in patients with FLT3-mutated (FLT3mut) AML who are not eligible for allogeneic transplantation.

Cyclin-dependent kinase inhibitors in malignant hematopoiesis

The cell-cycle is a tightly orchestrated process where sequential steps guarantee cellular growth linked to a correct DNA replication. The entire cell division is controlled by cyclin-dependent kinases (CDKs). CDK activation is balanced by the activating cyclins and CDK inhibitors whose correct expression, accumulation and degradation schedule the time-flow through the cell cycle phases. Dysregulation of the cell cycle regulatory proteins causes the loss of a controlled cell division and is inevitably linked to neoplastic transformation. Due to their function as cell-cycle brakes, CDK inhibitors are considered as tumor suppressors. The CDK inhibitors p16^INK4a and p15^INK4b are among the most frequently altered genes in cancer, including hematopoietic malignancies. Aberrant cell cycle regulation in hematopoietic stem cells (HSCs) bears severe consequences on hematopoiesis and provokes hematological disorders with a broad array of symptoms. In this review, we focus on the importance and prevalence of deregulated CDK inhibitors in hematological malignancies.

Inability to phosphorylate Y88 of p27Kip1 enforces reduced p27 protein levels and accelerates leukemia progression

The cyclin-dependent kinase (CDK) inhibitor p27^Kip1 regulates cell proliferation. Phosphorylation of tyrosine residue 88 (Y88) converts the inhibitor into an assembly factor and activator of CDKs, since Y88-phosphorylation restores activity to cyclin E,A/CDK2 and enables assembly of active cyclin D/CDK4,6. To investigate the physiological significance of p27 tyrosine phosphorylation, we have generated a knock-in mouse model where Y88 was replaced by phenylalanine (p27-Y88F). Young p27-Y88F mice developed a moderately reduced body weight, indicative for robust CDK inhibition by p27-Y88F. When transformed with v-ABL or BCR::ABL1^p190, primary p27-Y88F cells are refractory to initial transformation as evidenced by a diminished outgrowth of progenitor B-cell colonies. This indicates that p27-Y88 phosphorylation contributes to v-ABL and BCR::ABL1^p190 induced transformation. Surprisingly, p27-Y88F mice succumbed to premature v-ABL induced leukemia/lymphoma compared to p27 wild type animals. This was accompanied by a robust reduction of p27-Y88F levels in v-ABL transformed cells. Reduced p27-Y88F levels seem to be required for efficient cell proliferation and may subsequently support accelerated leukemia progression. The potent downregulation p27-Y88F levels in all leukemia-derived cells could uncover a novel mechanism in human oncogenesis, where reduced p27 levels are frequently observed.

Identification of a highly efficient dual type I/II FMS-like tyrosine kinase inhibitor that disrupts the growth of leukemic cells

Internal tandem duplications (ITDs) in the FMS-like tyrosine kinase-3 (FLT3) are causally linked to acute myeloid leukemia (AML) with poor prognosis. Available FLT3 inhibitors (FLT3i) preferentially target inactive or active conformations of FLT3. Moreover, they co-target kinases for normal hematopoiesis, are vulnerable to therapy-associated tyrosine kinase domain (TKD) FLT3 mutants, or lack low nanomolar activity. We show that the tyrosine kinase inhibitor marbotinib suppresses the phosphorylation of FLT3-ITD and the growth of permanent and primary AML cells with FLT3-ITD. This also applies to leukemic cells carrying FLT3-ITD/TKD mutants that confer resistance to clinically used FLT3i. Marbotinib shows high selectivity for FLT3 and alters signaling, reminiscent of genetic elimination of FLT3-ITD. Molecular docking shows that marbotinib fits in opposite orientations into inactive and active conformations of FLT3. The water-soluble marbotinib-carbamate significantly prolongs survival of mice with FLT3-driven leukemia. Marbotinib is a nanomolar next-generation FLT3i that represents a hybrid inhibitory principle.

Small Molecule Inhibitors of MERTK and FLT3 Induce Cell Cycle Arrest in Human CD8+ T Cells

There is an increasing interest in the development of Receptor Tyrosine Kinases inhibitors (RTKIs) for cancer treatment, as dysregulation of RTK expression can govern oncogenesis. Among the newer generations of RTKIs, many target Mer Tyrosine Kinase (MERTK) and Fms related RTK 3 (FLT3). Next to being overexpressed in many cancers, MERTK and FLT3 have important roles in immune cell development and function. In this study, we address how the new generation and potent RTKIs of MERTK/FLT3 affect human primary CD8⁺ T cell function. Using ex vivo T cell receptor (TCR)-activated CD8⁺ T cells, we demonstrate that use of dual MERTK/FLT3 inhibitor UNC2025 restricts CD8⁺ T proliferation at the G2 phase, at least in part by modulation of mTOR signaling. Cytokine production and activation remain largely unaffected. Finally, we show that activated CD8⁺ T cells express FLT3 from day two post activation, and FLT3 inhibition with AC220 (quizartinib) or siRNA-mediated knockdown affects cell cycle kinetics. These results signify that caution is needed when using potent RTKIs in the context of antitumor immune responses.

Pharmacological impact of FLT3 mutations on receptor activity and responsiveness to tyrosine kinase inhibitors

Acute myelogenous leukaemia (AML) is an aggressive blood cancer characterized by the rapid proliferation of immature myeloid blast cells, resulting in a high mortality rate. The 5-year overall survival rate for AML patients is approximately 25%. Circa 35% of all patients carry a mutation in the FLT3 gene which have a poor prognosis. Targeting FLT3 receptor tyrosine kinase has become a treatment strategy in AML patients possessing FLT3 mutations. The most common mutations are internal tandem duplications (ITD) within exon 14 and a single nucleotide polymorphism (SNP) that leads to a point mutation in the D835 of the tyrosine kinase domain (TKD). Variations in the ITD sequence and the occurrence of other point mutations that lead to ligand-independent FLT3 receptor activation create difficulties in developing personalized therapeutic strategies to overcome observed mutation-driven drug resistance. Midostaurin and quizartinib are tyrosine kinase inhibitors (TKIs) with inhibitory efficacy against FLT3-ITD, but exhibit limited clinical impact. In this review, we focus on the structural aspects of the FLT3 receptor and correlate those mutations with receptor activation and the consequences for molecular and clinical responsiveness towards therapies targeting FLT3-ITD positive AML.

Therapeutic targeting of FLT3 and associated drug resistance in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease caused by several gene mutations and cytogenetic abnormalities affecting differentiation and proliferation of myeloid lineage cells. FLT3 is a receptor tyrosine kinase commonly overexpressed or mutated, and its mutations are associated with poor prognosis in AML. Although aggressive chemotherapy often followed by hematopoietic stem cell transplant is the current standard of care, the recent approval of FLT3-targeted drugs is revolutionizing AML treatment that had remained unchanged since the 1970s. However, despite the dramatic clinical response to targeted agents, such as FLT3 inhibitors, remission is almost invariably short-lived and ensued by relapse and drug resistance. Hence, there is an urgent need to understand the molecular mechanisms driving drug resistance in order to prevent relapse. In this review, we discuss FLT3 as a target and highlight current understanding of FLT3 inhibitor resistance.

Design of Hydrazide-Bearing HDACIs Based on Panobinostat and Their p53 and FLT3-ITD Dependency in Antileukemia Activity

Here, we present a new series of hydrazide-bearing class I selective HDAC inhibitors designed based on panobinostat. The cap, linker, and zinc-binding group were derivatized to improve HDAC affinity and antileukemia efficacy. Lead inhibitor 13a shows picomolar or low nanomolar IC₅₀ values against HDAC1 and HDAC3 and exhibits differential toxicity profiles toward multiple cancer cells with different FLT3 and p53 statuses. 13a indirectly inhibits the FLT3 signaling pathway and down-regulates master antiapoptotic proteins, resulting in the activation of pro-caspase3 in wt-p53 FLT3-ITD MV4-11 cells. While in the wt-FLT3 and p53-null cells, 13a is incapable of causing apoptosis at a therapeutic concentration. The MDM2 antagonist and the proteasome inhibitor promote 13a-triggered apoptosis by preventing p53 degradation. Furthermore, we demonstrate that apoptosis rather than autophagy is the key contributing factor for 13a-triggered cell death. When compared to panobinostat, 13a is not mutagenic and displays superior in vivo bioavailability and a higher AUC_0-inf value.

CDK6 Inhibition: A Novel Approach in AML Management

Acute myeloid leukemia (AML) is a complex disease with an aggressive clinical course and high mortality rate. The standard of care for patients has only changed minimally over the past 40 years. However, potentially useful agents have moved from bench to bedside with the potential to revolutionize therapeutic strategies. As such, cell-cycle inhibitors have been discussed as alternative treatment options for AML. In this review, we focus on cyclin-dependent kinase 6 (CDK6) emerging as a key molecule with distinct functions in different subsets of AML. CDK6 exerts its effects in a kinase-dependent and -independent manner which is of clinical significance as current inhibitors only target the enzymatic activity.

MEK inhibition enhances the response to tyrosine kinase inhibitors in acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is a key driver of acute myeloid leukemia (AML). Several tyrosine kinase inhibitors (TKIs) targeting FLT3 have been evaluated clinically, but their effects are limited when used in monotherapy due to the emergence of drug-resistance. Thus, a better understanding of drug-resistance pathways could be a good strategy to explore and evaluate new combinational therapies for AML. Here, we used phosphoproteomics to identify differentially-phosphorylated proteins in patients with AML and TKI resistance. We then studied resistance mechanisms in vitro and evaluated the efficacy and safety of rational combinational therapy in vitro, ex vivo and in vivo in mice. Proteomic and immunohistochemical studies showed the sustained activation of ERK1/2 in bone marrow samples of patients with AML after developing resistance to FLT3 inhibitors, which was identified as a common resistance pathway. We examined the concomitant inhibition of MEK-ERK1/2 and FLT3 as a strategy to overcome drug-resistance, finding that the MEK inhibitor trametinib remained potent in TKI-resistant cells and exerted strong synergy when combined with the TKI midostaurin in cells with mutated and wild-type FLT3. Importantly, this combination was not toxic to CD34+ cells from healthy donors, but produced survival improvements in vivo when compared with single therapy groups. Thus, our data point to trametinib plus midostaurin as a potentially beneficial therapy in patients with AML.

Role of alternative polyadenylation dynamics in acute myeloid leukaemia at single-cell resolution

Alternative polyadenylation (APA) has been discovered to play regulatory roles in the development of many cancer cells through preferential addition of a poly(A) tail at specific sites of pre-mRNA. A recent study found that APA was involved in the mediation of acute myeloid leukaemia (AML). However, unlike gene expression heterogeneity, little attention has been directed toward variations in single-cell APA for different cell types during AML development. Here, we used single-cell RNA-seq data of a massive population of 16,843 bone marrow mononuclear cells (BMMCs) from healthy and AML patient samples to investigate dynamic APA usage in different cell types. Abnormalities of APA dynamics in the BMMCs from AML patient samples were uncovered compared to the stable APA dynamics in samples from healthy individuals, as well as lower APA diversity between eight cell types in AML patients. Genes with APA dynamics specific to the AML samples were significantly enriched in cellular signal transduction pathways that contribute to AML development. Moreover, many leukaemic cell marker genes such as NF-κB, GATA2 and IAP-Family genes exhibited APA dynamics that specifically affected abnormal proliferation and differentiation of leukemic BMMCs. Additionally, mature erythroid cells displayed greater APA dynamics and global 3' UTR shortening compared with other cell types. Our results revealed extensive involvement of APA regulation in leukemia development and erythropoiesis at the single-cell level, providing a high-resolution atlas to navigate cellular mRNA processing landscapes of differentiated cells in AML.

Targeting Oncogenic Signaling in Mutant FLT3 Acute Myeloid Leukemia: The Path to Least Resistance

The identification of recurrent driver mutations in genes encoding tyrosine kinases has resulted in the development of molecularly-targeted treatment strategies designed to improve outcomes for patients diagnosed with acute myeloid leukemia (AML). The receptor tyrosine kinase FLT3 is the most commonly mutated gene in AML, with internal tandem duplications within the juxtamembrane domain (FLT3-ITD) or missense mutations in the tyrosine kinase domain (FLT3-TKD) present in 30–35% of AML patients at diagnosis. An established driver mutation and marker of poor prognosis, the FLT3 tyrosine kinase has emerged as an attractive therapeutic target, and thus, encouraged the development of FLT3 tyrosine kinase inhibitors (TKIs). However, the therapeutic benefit of FLT3 inhibition, particularly as a monotherapy, frequently results in the development of treatment resistance and disease relapse. Commonly, FLT3 inhibitor resistance occurs by the emergence of secondary lesions in the FLT3 gene, particularly in the second tyrosine kinase domain (TKD) at residue Asp835 (D835) to form a ‘dual mutation’ (ITD-D835). Individual FLT3-ITD and FLT3-TKD mutations influence independent signaling cascades; however, little is known about which divergent signaling pathways are controlled by each of the FLT3 specific mutations, particularly in the context of patients harboring dual ITD-D835 mutations. This review provides a comprehensive analysis of the known discrete and cooperative signaling pathways deregulated by each of the FLT3 specific mutations, as well as the therapeutic approaches that hold the most promise of more durable and personalized therapeutic approaches to improve treatments of FLT3 mutant AML.

Role of p27Kip1 as a transcriptional regulator

The protein p27Kip1 is a member of the Cip/Kip family of cyclin-dependent kinase (Cdk) inhibitors. It interacts with both the catalytic and the regulatory subunit (cyclin) and introduces a region into the catalytic cleave of the Cdk inducing its inactivation. Its inhibitory capacity can be modulated by specific tyrosine phosphorylations. p27Kip1 also behaves as a transcriptional regulator. It associates with specific chromatin domains through different transcription factors. ChIP on chip, ChIP-seq and expression microarray analysis allowed the identification of the transcriptional programs regulated by p27Kip1. Thus, important cellular functions as cell division cycle, respiration, RNA processing, translation and cell adhesion, are under p27Kip1 regulation. Moreover, genes involved in pathologies as cancer and neurodegeneration are also regulated by p27Kip1, suggesting its implication in these pathologies. The carboxyl moiety of p27Kip1 can associate with different proteins, including transcriptional regulators. In contrast, its NH2-terminal region specifically interacts with cyclin-Cdk complexes. The general mechanistic model of how p27Kip1 regulates transcription is that it associates by its COOH region to the transcriptional regulators on the chromatin and by the NH2-domain to cyclin-Cdk complexes. After Cdk activation it would phosphorylate the specific targets on the chromatin leading to gene expression. This model has been demonstrated to apply in the transcriptional regulation of p130/E2F4 repressed genes involved in cell cycle progression. We summarize in this review our current knowledge on the role of p27Kip1 in the regulation of transcription, on the transcriptional programs under its regulation and on its relevance in pathologies as cancer and neurodegeneration.